NASA Technical Reports Server (NTRS) 19970026973: The Visible Imaging System (VIS) for the Polar Spacecraft

32 Pages·1.4 MB·English

by  NASA Technical Reports Server (NTRS)

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THE VISIBLE IMAGING SYSTEM (VIS) FOR THE POLAR
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L. A. FRANK, J. B. SIGWARTH, J. D. CRAVEN, J. P.CRAVENS, J. S. DOLAN,
M. R. DVORSKY, P. K. HARDEBECK, J. D. HARVEY and D. W. MULLER
Department ofPhysics andAstronomy, The University oflowa, Iowa City,IA 52242, U.S.A.
(Received 26 July, 1093)
Abstract. The Visible Imaging System (VIS) is a set of three low-light-level cameras to be flown
on the POLAR spacecraft of the Global Geospace Science (GGS) program which is an element of
the International Solar-Terrestrial Physics (ISTP) campaign. Two of these cameras share primary and
some secondary optics and are designed to provide images of the nighttime auroral oval at visible
wavelengths. A third camera is used to monitor the directions of the fields-of-view of these sensitive
auroral cameras with respect to sunlit Earth. The auroral emissions of interest include those from N+
at 391.4 nm, O Iat 557.7 and 630.0 nm, H 1at 656.3 nm, and OII at 732.0 nm. The two auroral cameras
have different spatial resolutions. These resolutions are about l0 and 20 km from aspacecraft altitude
of 8 Re. The time to acquire and telemeter a 256 x 256-pixel image is about 12 s. The primary
scientific objectives of this imaging instrumentation, together with the in-situ observations from the
ensemble of ISTP spacecraft, are (1) quantitative assessment of the dissipation of magnetospheric
energy into the auroral ionosphere, (2) an instantaneous reference system for the in-situ measurements,
(3) development of a substantial model for energy flow within the magnetosphere, (4) investigation
of the topology of the magnetosphere, and (5) delineation of the responses of the magnetosphere to
substorms and variable solar wind conditions.
1. Introduction
The first global images of Earth's aurora were acquired by three scanning photome-
ters, one each for emissions from N+ 391.4 nm, O !557.7 nm, and O !630.0 nm, on
board the low-altitude spacecraft ISIS 2(Anger e_ ol., 1973; Shepherd et al., 1973).
Although these images were obtained over the Northern Hemisphere only once per
orbit because orbital motion provided one of the dimensions for the pixel array of an
image, the potential value of global images for organizing and interpreting in-situ
observations of particles and fields was clearly demonstrated. Indeed the ability to
obtain global images provided considerable excitement in the scientific community
from the visualization of the overall morphology of the auroral emissions (Anger
et al., 1974). The ISIS-2 observations were soon followed by the availability
of broad-band images from similarly scanning radiometers, 400-1130 nm, from
the Defense Meteorological Satellite Program (DMSP) spacecraft (Rogers et _d.,
1074). Because the ISIS-2 and DMSP satellites orbited at low-altitudes, in the
range of 1000 km, typically only a portion of the auroral oval was viewed during
a single polar pass. These spacecraft were later followed by the launch of Kyokko
into an orbit with apogee of about 4000 km. Kyokko was equipped with an image-
memory tube and obtained global auroral images at far-ultraviolet wavelengths,
Space Science Reviews 71: 297-328, 1995.
(E) 1995 Khtwer Academic Publishers. Printed in Belgium.
298 L. AFRANK El" AL.
in this case within a broad passband at about 120 to 140 nm, with unprecedented
temporal resolution (Kaneda et aI., 1977). The frame repetition period was 2 min.
In the 1980s auroral imagers were launched with several spacecraft. These space-
craft were Dynamics Explorer 1(Frank et al., 1981), HILAT (Meng and Huffman,
1984), Viking (Anger et al., 1987), Polar BEAR (Meng et al., 1987), and Ake-
bono (Kaneda and Yamamoto, 1991). HILAT and Polar BEAR were launched into
low-altitude orbits and thus yielded an auroral image once per orbital period, about
100 min. However, their imaging equipment included spectrometers with sufficient
resolution toseparate the O 1130.4-nm and 135.6-nm emissions. Such spectroscopy
is important in quantitative evaluation of the spectrum of precipitating electrons
(Strickland et al., 1983). The scanning spectrometer on Polar BEAR also provided
images of the nighttime auroral oval at visible wavelengths. The apogee of the
Viking orbit was located at higher altitudes, about 13500 km. The two cameras,
one for the O I emissions at 130.4 and 135.6 nm with passband extending into
the longer wavelengths of N2 Lyman-Birge-Hopfield (LBH) emissions and the
other for the LBH emissions, were capable of frame repetition periods of 20 s.
These Viking images were very useful in studies of the temporal evolution of the
spatial distribution of features at auroral and polar cap latitudes (Murphree et al.,
1987).
The three spin-scan photometers on Dynamics Explorer 1 (DE 1) provided
hundreds of thousands of global auroral images during the nine years following their
initial operational turn-on in fall of 1981 (Frank and Craven, 1988). Two of these
imagers viewed the aurora at visible wavelengths in narrow passbands as selected
with a filter wheel. The third imager was equipped with broad-band filters for far-
ultraviolet wavelengths inthe range of ,_ 120 to 175 nm. The orbit was sufficiently
high, an altitude of about 22 000 km, that the viewing time of the entire auroral oval
during a single orbit was 2 to 3 hr. Such viewing times allowed continuous global
viewing of the development of entire auroral substorms, i.e., through growth, onset,
expansion and recovery phases. The imagers for visible wavelengths are the first,
and at present only, optical systems that have successfully viewed the dim auroral
emissions in the nighttime atmosphere with the intense emissions from sunlit Earth
in the field-of-view. These imagers were usually operated in a mode such that an
image frame with afield-of-view 30°x 30°divided into 14400 pixels was acquired
once each 720 s.
The Visible Imaging System (VIS) for the Polar spacecraft isdesigned toachieve
high-time and -spatial resolution images of the nighttime polar and auroral emis-
sions at visible wavelengths. There is an ancillary camera for far-ultraviolet wave-
lengths within a broad passband, 124-149 nm. This camera can provide full images
of Earth from radial distances > 5.8 Re and is used to verify the proper pointing
of a two-axis targeting mirror for the two primary cameras for visible wavelengths.
The optics for the visible cameras is based upon the off-axis catoptric design with
super-polished surfaces that was successfully used for the DE-1 spin-scan imagers.
Because the VIS is mounted on a despun platform and can stare at Earth its per-
THE VISIBLE IMAGING SYSTEM 299
formance in terms of angular resolution and frame rate can be greatly improved
relative to that for the serial single-pixel sampling on the rotating DE 1. For exam-
ple, consider the viewing of the nighttime auroral zone from a Polar spacecraft
altitude of 7.4 Re and high latitude. The targeting mirror for the cameras for visible
wavelengths can be used to position the fields-of-view of these cameras such that
viewing of the auroras is optimized. For the low-resolution camera its field-of-view
is 5.6° x 6.3° and sufficient to usually include the entire nighttime auroral oval.
Within this field- of-view a frame of 65500 pixels can be telemetered every 12 s.
For a DE-1 image taken at this altitude the corresponding frame of 576 pixels
could be telemetered every 144 s. The counts/pixel for a given auroral brightness
are similar for the two images.
The scientific objectives for observations with VIS can be grouped into five pri-
mary categories: (1) quantitative assessment of the dissipation of magnetospheric
energy into the auroral and polar ionospheres, (2) an instantaneous reference sys-
tem for the in-situ measurements with the ISTP spacecraft, (3) development of a
substantial model for energy flow within the magnetosphere, (4) investigation of
the topology of the magnetosphere, and (5) delineation of the responses of the mag-
netosphere to substorms and variable solar wind conditions. It should be realized
that these general objectives cannot be achieved without the in-situ observations
from the various ISTP spacecraft. Much has been learned from previous studies
as to the specific investigations that will contribute to these general objectives.
Because this paper is devoted to a description of the instrumentation we limit our
discussion of the objectives to an illustrative example for each category.
In order to achieve (1) above interleaved sequences of images of the emissions
from N+ at 391.4 nm and for Olat 630.0 nm are acquired. The 391.4-nm emission
is a good measure of the electron energy flux into the atmosphere and the ratio
of the two intensities is a measure of the electron energy spectral index. The
complication for determination of the energy fluxes and electron spectra in this
manner is caused by the reflectance of Earth's surface and, if present, clouds.
This is basically a tractable radiative transfer problem (Rees et al., 1988). The
reflectance of Earth and clouds and their contributions to the observed intensities
are to be evaluated in part with images at filter wavelengths that are offset from
auroral emission lines. The determination of electron energy spectra with the visible
emission lines is complementary tothat achieved at far-ultraviolet wavelengths and
can be applied for lower electron energies than the latter measurement (Rees et al.,
1988; Strickland ctal., 1983). Of course, the visible observations can be only taken
for the nighttime aurora where most of the precipitating charged particle energy
fluxes occur whereas the far-ultraviolet measurements are possible for the sunlit
atmosphere.
The provision of an instantaneous coordinate system, category (2), is obvious.
The global auroral images place the in-situ observations in the context of auroral
substorm phase or other activity and of geographical location of the imprint of
charged particle precipitation. For example, measurements of particles and fields
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L.A FRANK ET AL.
in the distant polar magnetosphere during periods that a theta aurora (Frank et al.,
1986) is observed can resolve the controversy as to whether the transpolar arc
of this auroral configuration is the footprint of bifurcation of the magnetospheric
lobes (Frank, 1988) or large-scale spatial distortion of the plasma regimes in the
magnetotail (Akasofu and Roederer, 1984; Lyons, 1985).
Category (3) studies of the gross flow of energy within the magnetosphere
extend over a broad range, including the inference of the total magnetic energy
in the magnetotail from the area poleward of the auroral oval and the relative
motions of the ion and electron plasmas in the vicinity of the inner edge of the
electron plasma sheet. In order to obtain the footprint of protons precipitating
into the ionosphere the VIS is equipped with a narrow-band filter for HI 656.3-
nm emissions. With interlaced images of Of 557.7-nm emissions the large-scale
interrelationship between precipitation of electrons and protons from the near-Earth
plasma sheet into the atmosphere can be studied.
The mapping of plasma boundaries into features of the auroral luminosities
is important for extending in-situ observations of these boundaries with a single
spacecraft into a visualization of their geometries and temporal evolutions. Such
studies are included in category (4). It is clear that such identification of these
boundaries can contribute significantly to our knowledge of the magnetic field
topology of the magnetosphere. Only limited studies of this type have been reported.
One of the notable examples is the identification of poleward discrete arcs in the
auroral oval with the plasma sheet boundary layer from simultaneous observations
with the DE-1 imager and an ISEE-2 plasma analyzer (Frank and Craven, 1988) and
with magnetometers on both spacecraft (Elphic et al., 1988). The comparison of
images from the Polar spacecraft and in-situ fields and particles observations with
both the Polar and Geotail spacecraft should substantially increase our knowledge
of the magnetic topology of the magnetosphere and its relationship to major plasma
regions.
Analysis of DE-1 image sequences for small, isolated substorms has revealed
that the polar cap area, i.e., that area enclosed by the poleward edge of the auro-
ral oval, responds to the southward turning of the interplanetary magnetic field
(Frank and Craven, 1988; Frank, 1988). Although previous studies with low-
altitude observations indicated that this response occurs (Meng and Makita, 1986)
the DE-1 images provided determination of the entire polar cap boundary with
sufficient temporal resolution, 12 min, to clearly identify this effect. These studies
are part of the general topic (5) above. The DE-1 results showed that the polar cap
area expands when the interplanetary field turns southward and increases until a
substorm onset occurs. During the expansion phase the polar cap area decreases.
The expansion and subsequent contraction of polar cap area can be interpreted as
the storage and release, respectively, of the total magnetic energy in the magneto-
tail lobes. This energy can be quantitatively estimated with simple models of the
magnetotail magnetic fields (Coroniti and Kennel, 1972). The question remains as
to the precise connection between the polar cap area and open magnetic field lines
THE VISIBLE IMAGING SYSTEM 301
in the lobes. Detailed analysis of auroral images and simultaneous fields and par-
ticles measurements with the Polar spacecraft should resolve this issue and refine
the estimates of the transport of solar wind energy into the magnetotail and its
explosive release during substorms.
The opportunity to construct and launch a state-of-the-art camera does not
occur very often. With the addition of a few filters into the instrumentation the
objectives can address several targets of opportunity. The two filters at 317.3 and
360.1 nm provide high spatial resolution for the total columnar ozone in Earth's
sunlit atmosphere. One of these filters can be used to acquire global monitoring
of the occurrence of lightning. A narrowband filter at 589.0 nm is also included
for surveys of the Moon's Na cloud (Mendillo et al., 1991) and Na emissions in
Earth's nighttime atmosphere. The filter for OH emissions at 308.5 nm can be used
to pursue the topic of atmospheric holes by a search for clouds of OH above and
in the upper atmosphere (Frank and Sigwarth, 1993).
2. Overview of Instrumentation
A photograph of the instrumentation is shown in Plate 1. The instrument is
to be mounted on the despun platform of the Polar spacecraft. The top covers
are removed. These covers are radiators equipped with optical surface reflectors
(OSRs) for passively cooling the charge-coupled devices (CCDs) and the electron-
ics. A diagram of the instrument as viewed from this top side is shown in Figure 1
and is useful in identifying the various subsystems that are visible in Plate 1. Two
major compartments are visible in Plate 1, the optics section on the left-hand side
and the electronics section on the right. The gold-colored aperture door and two
rectangular collimators extend forward of the main optics section. The motor-
driven door provides protection for the internal optics against particulate material
and condensables during launch and inflight reorientations of the spacecraft spin
axis. The instrument is assembled in a Class-100 clean room. The rectangular
collimator on the left-hand side services the two cameras for auroras at visible
wavelengths. The field-of-view provided by this collimator is 20° x 20°. These
two cameras share primary optics and some of the secondary optics. The cameras
are nearly identical with the major exception of angular resolution. The angular
resolution of the medium-resolution camera is0.011 ° x 0.013 ° (pixel size) and that
of the low-resolution camera is0.022 ° x 0.025 °. The instantaneous fields-of-view
of these cameras is significantly less than that provided by the collimator. Thus a
bi-axially rotated mirror is employed to cover this entire field-of-view by mosaicing
images, particularly at low altitudes. An overview of the performance parameters
for the cameras is given in Table I. At the primary focal plane a field-stop wheel
is used to block the image of sunlit Earth from the secondary optics. This field
stop wheel thusly prevents this intense light from direct entry into the secondary
optics. The optical path is then folded in a complex geometry to accommodate the
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TABLE l
Overview of the visible imaging system (VIS)
Size 61 x 66 x 25 cm
Mass 28.65 kg
Power 30 W
Image frame rate 12s
Telemetry 11 kilobits s- 1
Number of pixets/frame 256 x 256 pixels
Aurora cameras Low resolution Medium resolution
Field-of-view
-Overall 20° x 20° 20° × 20°
-Instantaneous 5.4" × 6.3 ° 2.8° x 3.3°
Passband width (12 filters) _ 1nm _ I nm
Wavelength range 308-732 nm 308-732 nm
Sensitivity (630 nm) 3 counts kR-1 pixel-i 0.8 count kR-1 pixel-1
Time resolution 12s 12 s
Angular resolution 0.02 ° 0.0l °
Spatial resolution (at Earth's surface as seen from the apogee altitude of 8 Re)
20 km 10 km
Earth camera
Field-of-view 20° x 20°
Passband width _25 nm
Wavelength range 124-149 nm
Sensitivity 7.2 counts kR- Jpixel-
Time resolution 12 s
Angular resolution 0.08 °
Spatial resolution (at Earth's surface as seen from the apogee altitude of 8 R,_)
70 km
allowed dimensions of the housing. A plane mirror rotated by a motor is used to
determine which of the two cameras receives the image. The light is collimated
and passed through a selected narrow-band filter (the gold-colored wheel) and the
image isreformed at the faceplate of an image intensifier. The intensified image is
then optically transferred to a CCD. This mechanical isolation allows the cooling
of the CCD to temperatures in the range of -90° C in order to obviate the delete-
rious effects of damage from energetic ions in the inner radiation zone. At these
temperatures, the electrons associated with the displacement defects are trapped.
The small gold-colored 'knobs' provide the heat strap connection to the topside
radiator.
THE VISIBLE IMAGING SYSTEM 303
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Plate 1. Photograph of the Visible Imaging System (VlS) without the top radiator plates.
The smaller rectangular collimator behind the right-hand side of the door is
used for the Earth camera. This Earth camera provides a 20° x 20° image without
the need for mosaicing several images. The angular resolution is lesser than that
of the visible cameras, 0.08 ° x 0.08 °. For comparison the corresponding pixel for
the imagers on DE-1 is circular with angular diameter 0.25 °. The Earth camera is
equipped with one broad-band filter at far-ultraviolet wavelengths. These images
are processed within the VIS toascertain that no intense light sources, such as sunlit
Earth, are viewed by the secondary optics of the visible cameras. It is possible to
telemeter one image from the Earth camera each 12 s.
The electronics compartment is on the right-hand side of the instrument as
viewed in Plate 1. The electronics stack in the rear of this compartment is three
identical sets of power supplies and control electronics for the three sensors. The
front stack includes the six microprocessors with a total of 736 kbytes of memory
for operating the instrument and for data compression. The two primary power
convertors are located out-of-sight below the sensor electronics.
THE VISIBLE IMAGING SYSTEM 305
3. Mechanical Design
The VIS mechanical structure consists of three major subassemblies; the optics
housing, the electronics housing, and the radiator. In addition the various motors
that are employed in the instrument are described in this section.
The optics housing is designed to provide a stiff, stable optics platform for
the two visible cameras and the Earth camera. This housing is required to be
light-tight and to be of minimum mass. The optics housing assembly includes the
optical bench, optical subassemblies, and the collimator extension subassemblies.
The optical subassemblies are mounted to a pocketed beryllium base plate that
serves as the optical bench. The structure is optimized for the reduction of thermal
gradients.
The optical bench is enclosed by light-tight, pocketed Mg walls. Each interface,
i.e., optics wall-to-bench and optics wall-to-wall, is baffled and sealed with O-
rings. To prevent the possibility of distortion of the optical bench by the difference
in the thermal expansion coefficients of Be and Mg, attach points are designed to
eliminate fastener interference.
The collimator extensions include the Earth-camera collimator, the collimator
for the two visible cameras, and an aperture door for the collimators. Each colli-
mator is machined from Mg stock and subsequently etched to minimize mass. The
baffles inthe collimators are coated with Martin Black. Each of the two collimators
is thermally isolated from the optics housing. A special light-tight seal is used to
prevent entry of stray light into the optics housing, without metal-to-metal contact
at this interface. The aperture door is machined from a thick Mg plate to produce
a stiff, low-mass structure.
The electronics housing is fabricated from Mg and is attached to the optics hous-
ing by a light-tight interface wall which also accommodates the electrical interface
between the optics and electronics sections. Because this housing interfaces with
the optics housing, the radiator, and the spacecraft platform, careful consideration
isgiven to the structural dynamics of these interfaces in order to prevent distortion
of the optical bench.
The instrument thermal radiator consists of two thermally isolated plates that are
covered with OSRs. One of these radiators isused to dispose of the thermal energy
from the power dissipation in the optics and electronics sections and to maintain
the temperature of the optical bench in the range -20 to 0 °C. The larger radiator
is used to passively cool the CCDs to -90 °C. This radiator is coupled directly
to the tantalum radiation shield surrounding each CCD by means of a copper heat
strap.
There are six stepping motors and one wax motor in the VIS. The six motors
are manufactured by Schaeffer Magnetics, Inc. and are two similar types. The first
type isused for the bi-axial targeting mirror (two motors), the sensor select mirror,
and the collimator door. The second design is used for the filter wheel and the
field stop wheel. Both motors are similar except that the motor step for the first
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L. A FRANK ET AL.
type is 3.75 ° and is reduced by an integral 80 : 1harmonic drive, and the motor
step for the second type is 1.5° with no harmonic drive. Both motor types operate
efficiently at 100 steps s-1 and have the capability of bidirectional rotation. The
two motors for the bi-axial targeting mirror provide motion of the fields-of-view
for the visible cameras with respect to the platform in two orthogonal planes, i.e.,
in a plane parallel to the spacecraft spin axis and in a plane perpendicular to this
axis. The increments in the field-of-view are 0.094 ° ineither direction. The Starsys
wax motor is used for a one-shot mechanism that permanently opens the collimator
door in the event that the door motor is not operable.
4. Optics for the Low- and Medium-Resolution Cameras
The Visible Imaging System (VIS) was designed with a stringent set of specifica-
tions in order to image the dim nightside auroras with the bright dayside of Earth
within the instrument's field-of-view. The VIS isan f/8 re-imaging system with an
intermediate primary focal plane and final focal planes at the sensors. The camera
employs off-axis parabolic and fiat mirrors that provide the image at either the
low-resolution or medium- resolution sensors.
Exploded views of the VIS in the low-resolution and medium-resolution con-
figurations are shown in Figures 2 and 3, respectively. Light enters the collimator
assembly and is reflected by flat mirror M1 which is mounted on a bi-axial tar-
geting mechanism. The targeting mechanism is driven by two motors. Operation
of this bi-axial targeting mirror assembly permits the acquisition of 5.4° x 6.3°
or 2.8° x 3.3° images at any position within the 20° x 20° field-of-view of the
collimator for the low-resolution and medium-resolution cameras, respectively. An
aperture stop is placed between M1 and the off-axis parabolic reflector M2. The
aperture stop is also the entrance pupil with a diameter of 2.0 cm. M2 provides
the primary image at the position of the field stop wheel. The selection of field
stops on the wheel allows imaging near Earth's terminator, i.e., rejection of those
portions of the primary image with sunlit Earth from entering the secondary optics.
In the secondary optics the light is reflected by an angle of ,--,41° by the off-axis
parabolic surface of M3 and becomes nearly collimated. Sensor select mirror M4
is used to direct the light into the optical path for either the low-resolution or
medium-resolution sensors.
For the low-resolution sensor the nearly collimated light passes through a Lyot
stop and subsequently through one of twelve filters in the filter wheel. Off-axis
parabolic reflector M5 and the plane turning-mirror M6 present the focused image
at the low-resolution sensor (see Figure 2). For the medium-resolution sensor
the nearly collimated light from M4 passes through another Lyot stop and then
through the filter wheel (see Figure 3). Off- axis parabolic reflector M7 and inverted
parabolic reflector M8 provide the image at the medium-resolution sensor. The
inverted parabolic reflector M8 magnifies the image by a factor of two relative to